Coupled chemical-thermal solar power system and method

ABSTRACT

A CSP system is disclosed which couples a thermal and a chemical energy pathway. The thermal pathway utilizes a heat transfer fluid to collect concentrated sunlight as thermal energy at medium temperature and transfer this energy to a thermal-to-electric power cycle. In parallel, the chemical pathway uses a redox material which undergoes direct photoreduction in the receiver to store the solar energy as chemical potential. This redox material is then oxidized at very high temperatures in the power cycle in series with the thermal pathway heat exchanger. This coupling allows the receiver to perform at the high efficiencies typical of state of the art thermal power towers while simultaneously achieving the power cycle efficiencies typical of natural gas combustion plants and achieving a very high overall solar-to-electric conversion efficiency.

TECHNICAL FIELD

The embodiments disclosed herein include systems and methods in thefield of concentrating solar power (“CSP”) generation, also known assolar thermal power generation. The disclosed systems and methodsgenerally utilize two coupled parallel energy pathways, one thermal andone chemical, to convert solar energy to electrical energy at highefficiency. Specifically, the disclosed embodiments include a solarreceiver in communication with separate chemical energy storage materialand heat transfer fluid flowing or transported in separate pathways. Thechemical energy storage material undergoes low temperaturephotoreduction at the receiver. In addition, heat transfer fluid (“HTF”)is heated to an operational temperature at the solar receiver. Thechemical energy storage material and HTF are used to drive a power cyclewhich operates at relatively high temperatures because the chemicalenergy storage material is exothermically oxidized as, or in sequencewith, the HTF being cooled.

BACKGROUND

Concentrating solar technologies may generally be divided into thermalsystems for electric power generation and chemical systems for fuelsproduction and chemical processing. Variations on thermal CSP plants areknown in the art which utilize different types of reflectorconfigurations such as troughs, dishes, and heliostat fields. Known CSPsystems utilize many alternative heat transfer fluids such as oils,molten salts, and steam and can be used to drive various power cyclessuch as steam Rankine, supercritical steam Rankine, and supercriticalcarbon dioxide Brayton cycles.

The current state of the art in high-temperature CSP towers isrepresented by direct steam generation towers such as shown in US PatentApplication 2008/0302314 and molten nitrate salt towers such as shown inUS Patent Application 2008/0000231. These types of towers typicallyoperate at temperatures up to about 600° C. Greater power generationefficiency could be achieved with operational temperatures in excess of600° C. It is difficult to achieve operational temperatures above 600°C. with conventional CSP strategies.

Concentrating solar towers for driving chemical reactions have beensuggested in several forms. One known concept uses concentrated sunlightto generate heat to decompose biomass, such as described in US PatentApplication 2010/0249468. Another known method features the use ofconcentrated sunlight to cause water to undergo photolysis throughinteraction with catalysts, such as described in U.S. Pat. No.4,045,315. Other technologies use concentrated sunlight and areduction/oxidation cycle to create hydrogen gas from water or carbonmonoxide gas from carbon dioxide, such as described in US PatentApplication 2009/0107044. The foregoing chemical methods are notparticularly well suited for the generation of electrical power usingknown power turbine based power cycles.

Maximizing the efficiency of a power plant for a concentrating solarpower system is of great importance because it causes a reduction inoverall system capital cost by requiring a smaller solar field andreceiver for the same net energy production. In a concentrated solarpower tower, the overall solar-to-electric efficiency is the product ofthe solar field efficiency, the receiver (solar-to-thermal) efficiency,the storage efficiency, and the power cycle (thermal-to-electric)efficiency. The thermal-to-electric conversion system is very similar tofossil fuel systems at comparable temperatures, however, the conversionefficiency of a solar power cycle is typically much less than that of acombined cycle gas plant due to the lower operational temperatures.

The embodiments disclosed herein are directed toward overcoming one ormore of the problems discussed above.

SUMMARY OF THE EMBODIMENTS

The embodiments disclosed herein include concentrating solar power (CSP)systems and methods which couple a thermal and a chemical energypathway. The thermal pathway utilizes a heat transfer fluid to collectconcentrated sunlight as thermal energy at medium temperature andtransfer this energy to a thermal-to-electric power cycle. In parallel,the chemical pathway uses a redox material which undergoes directphotoreduction in the receiver to store the solar energy as chemicalpotential. This redox material is then oxidized at very hightemperatures in the power cycle in series with the thermal pathway heatexchanger. This coupling allows the receiver to perform at the highefficiencies typical of state of the art thermal power towers whilesimultaneously achieving the power cycle efficiencies typical of naturalgas combustion plants and achieving a very high overallsolar-to-electric conversion efficiency.

One disclosed embodiment is a CSP system comprising a solar receiverconfigured to receive concentrated solar flux and a quantity of heattransfer fluid (HTF) in thermal communication with the solar receiversuch that concentrated solar flux heats the HTF. The system alsoincludes a heat exchanger in thermal communication with the HTFproviding for heat exchange between the HTF and the working fluid of apower generation cycle. In addition, the system also includes a chemicalenergy storage material flowing in a chemical pathway coupled to thethermal pathway. The chemical energy storage material is also incommunication with the solar receiver such that concentrated solar fluxreduces a quantity of the chemical energy storage material in thereduction portion of an oxidation-reduction reaction. Thus, the chemicalenergy storage material can be alternatively referred to as a redoxmaterial.

The system further includes an oxidizer in communication with thechemical energy storage material, the oxidizer providing for theexothermic oxidation of the chemical energy storage material and furtherproviding for heat exchange between the chemical energy storage materialand the working fluid of the power cycle. Thus, the system utilizesparallel energy pathways, one thermal and one chemical. The use of twopathways coupled at the solar receiver results in a high-efficiency CSPplant.

The system may further include thermal energy storage operativelyassociated with a HTF conduit. In addition, the system may includeseparate chemical energy storage including a reduced chemical storagesystem operatively receiving reduced chemical energy storage materialfrom the solar receiver; and/or an oxidized chemical storage systemreceiving oxidized chemical energy storage material from the oxidizer.

An alternative embodiment disclosed herein comprises a power generationmethod having certain steps which may be performed in any suitable orderand which typically will be performed in a cyclical fashion. The methodembodiments are initiated by providing a solar receiver configured toreceive concentrated solar flux. HTF of any suitable type is flowed,transported or otherwise brought into thermal communication with thesolar receiver where the HTF is heated with the concentrated solar flux.The heated HTF is then flowed or transported from the solar receiver toa heat exchanger in a heat transfer fluid conduit. In the heat exchangerheat is exchanged between the heated heat transfer fluid and the workingfluid of a power cycle.

In a parallel cycle, a chemical energy storage (redox) material incommunication with the solar receiver is irradiated by the concentratedsolar flux thereby causing a quantity of the chemical energy storagematerial to be reduced. The reduced chemical energy storage material isthen flowed or transported between the solar receiver and an oxidizerelement. In the oxidizer, the chemical energy storage material isoxidized causing the release of heat energy. The released heat energy isexchanged with the working fluid of the power cycle. Power may then begenerated with the heated working fluid of the power cycle,

The disclosed embodiments all feature dual thermal and chemical energypathways. The embodiments may be implemented in any type ofconcentrating solar power apparatus and with any type of powergeneration cycle or cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified system schematic diagram illustrating a prior artCSP system.

FIG. 2 is a simplified system schematic diagram illustrating oneembodiment of a system having thermal and chemical energy pathways asdescribed herein.

FIG. 3 is a schematic diagram illustrating a redox cycle.

FIG. 4 is a simplified power cycle schematic illustrating arepresentative power cycle suitable for implementation with the systemsdisclosed herein.

FIG. 5 is simplified receiver schematics illustrating how the coupledpathways disclosed herein reduce radiative losses.

FIG. 6 is a simplified system schematic diagram illustrating analternative received design.

FIG. 7 is a flow chart representation of a representative method asdisclosed herein.

DETAILED DESCRIPTION

Unless otherwise indicated, all numbers expressing quantities ofingredients, dimensions reaction conditions and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about”.

In this application and the claims, the use of the singular includes theplural unless specifically stated otherwise. In addition, use of “or”means “and/or” unless stated otherwise. Moreover, the use of the term“including”, as well as other forms, such as “includes” and “included”,is not limiting. Also, terms such as “element” or “component” encompassboth elements and components comprising one unit and elements andcomponents that comprise more than one unit unless specifically statedotherwise.

As noted above, known CSP systems generally operate at significantlyless overall efficiency when compared to a combined cycle gas powergeneration plant. The reduced efficiency of a CSP system is primarilydue to lower operational temperatures. To maximize overall solar plantefficiency, it would be advantageous to use the high-efficiency powercycles available to combined cycle gas systems with a CSP plant,assuming this could be accomplished without compromising the balance ofthe CSP plant. This goal cannot be accomplished in state-of-the-artsolar power towers, such as nitrate salt towers or superheated steamtowers. In these towers, the energy pathway is strictly thermal, meaningthat all the energy transferred from the receiver to the power cycle istransferred as thermal energy. A representative energy flow in aconventional CSP system is illustrated in FIG. 1. Solar radiation isconcentrated by the solar field. The concentrated solar radiation istransferred as thermal energy to the receiver and captured with anintermediate heat transfer fluid. Thermal energy is then stored as hotstock heat transfer fluid in large tanks. When needed, the hot heattransfer fluid is sent to the power cycle via a heat exchanger. Finally,thermal energy is converted into electricity in a thermodynamic powercycle.

In known CSP plant configurations, the factors driving the receiver andpower cycle efficiencies are fundamentally counteracting one another.The efficiency of the power cycle increases with the inlet temperatureof the heat transfer fluid. However, the efficiency of the solarreceiver goes down with increasing temperature due to higher convectiveand radiative heat losses. In addition, the receiver has absolutetemperature limits because of salt decomposition and receiver materialconstraints. Because the power cycle and receiver temperatures arefundamentally coupled in a plant with only a thermal energy pathway,very high overall system efficiencies cannot be achieved.

The system and method embodiments disclosed herein utilize parallelenergy pathways, one thermal and one chemical in a high-efficiencyconcentrated solar power (CSP) plant. As shown in FIG. 2, arepresentative system 100 includes one or more thermal pathways 102consisting of a heat transfer fluid (HTF) such as steam/water, molten orsolid salt, molten or solid metal, oil a phase change material or othersuitable HTF in thermal communication with a solar receiver 104. Thesolar receiver 104 is typically associated with a central receiver towerand receives concentrated solar flux reflected by a field of heliostats.The methods disclosed herein could be implemented with other CSP designshowever, including but not limited to parabolic trough, linear Fresnel,and dish/engine systems.

HTF heated at the solar receiver 104 is flowed or transported to a heatexchanger 106 in a heat transfer fluid conduit 108. It is important tonote that although the conventional terminology of heat transfer fluid(HTF) is used herein, the system and methods may be implemented with aliquid, solid, gaseous or phase-changing HTF. Thus, the heat transferfluid conduit 108 may be a system of pipes or ducts and valves suitablefor the control of fluid flow, or the heat transfer fluid conduit 108may be any type of system suitable for transporting solids. The heattransfer fluid conduit 108 may include some fluid flow sections and somesolid transport sections.

In the heat exchanger 106, thermal energy is exchanged between the HTFand the working fluid of a power cycle. The heat exchanger(s) may be ofany type or any level of sophistication needed to provide for heatexchange between the HTF and a power generation cycle working fluid. Theheat exchanger 106 and other subsystems are, for technical convenience,described and shown in the figures as simple schematic elements. Allelements of a commercial system would be implemented with more complexapparatus. As generally shown in FIG. 2, the heated working fluid drivesa power generation cycle 110. Accordingly, the working fluid is, eitherdirectly or through an intermediate power cycle fluid, converted tomechanical energy and then electrical energy.

The system 100 and methods disclosed herein also include a parallelchemical energy pathway which includes a chemical energy storagematerial which undergoes reversible reduction and oxidation reactions(alternatively referred to herein as a “redox material”). In particular,the redox material is reduced in the receiver 104 and oxidized in anoxidizer 112. The oxidizer or an associated apparatus also provides forheat exchange with the working fluid of the power cycle 110. The redoxmaterial is flowed or transported between the receiver and oxidizer in achemical energy storage material conduit 114 which may be configured forfluid flow or solid transport as described above with respect to the HTFconduit 108.

As noted above, the redox material is directly photoreduced by the highconcentration of incident photons in the receiver 104, thus, the redoxmaterial stores the absorbed electromagnetic energy as a chemicalpotential. In the oxidizer 112, the redox material is oxidized, therebyreleasing high temperature thermal energy. A representative diagram ofthis type of chemical process is shown in FIG. 3 and described below. Itis important to note that the oxidizer element 112 will typically beimplemented with apparatus of significantly more complexity than isshown on FIG. 2. For example, the oxidizer 112 may include separateoxidation chambers, air or gas supplies, fluidized bed, heat exchangerand other elements.

Typically, CSP systems achieve a certain level of efficiency whenimplemented, for example, with current state of the art steam or moltensalt receivers. In addition, power plants implemented with combustivepower cycles have very good performance (for example, combined cyclenatural gas plants). The coupled thermal-chemical architecture describedherein allows a CSP system to take advantage of both power generationtechnologies without any fossil fuel consumption or environmentallyharmful emissions. Thus, the disclosed systems and methods have anadvantage over known state-of-the-art CSP plants in increasedthermal-to-electric conversion efficiency due in part to the hightemperature of the oxidation process. For example, typical steam ormolten salt CSP based power generation plants achievethermal-to-electric efficiencies of 40-44%. As shown in FIG. 2 anddetailed below, the disclosed systems and methods can achievetemperatures suitable to drive a power generation system having overallefficiencies of approximately 60% which are much closer to theefficiencies exhibited by combined cycle natural gas plants

The chemical energy pathway described above represents new systemarchitecture in the CSP industry. Whereas a thermal pathway transfersenergy by heating and cooling a heat transfer fluid, a chemical pathwaytransfers energy by storing energy in a material via an endothermicreaction and releasing it in an exothermic reaction. As noted above, thechemical pathway will consist of a material undergoing reversiblereduction and oxidation reactions. For illustration purposes, onepotential set of reactions is shown in FIG. 3, although the embodimentsdisclosed herein can be implemented with many alternative redoxmaterials.

As schematically illustrated in FIG. 3, a representative redox cyclefeatures a reduction step (top box) which takes place in the solarreceiver. A photon hits the oxidized material and breaks it into areduced material and free oxygen. This step depends only on the photondirectly supplying energy to break the bond between the metal and oxygenatoms. The reduced material (MnO in this example) is transferred to astorage tank. When it is needed, it is transferred to the power cyclewhere it is burned in oxygen releasing heat and closing the loop byrecreating the original oxidized material.

The foregoing representative redox process is governed by the balancebetween the energy of the chemical bonds and the energy of the photon.The bond energies are typically described in terms of Gibbs free energy,AG, and the energy required to drive an endothermic reaction or theenergy released by an exothermic reaction can be calculated withEquation 1.

ΔG _(reaction)=Σcoeff_(products) ΔG_(products)−Σcoeff_(reactants)ΔG_(reactants)   Equation 1

Reference AG values can be obtained from chemistry texts, NISTdatabases, or other sources. The coefficients come from the balancedchemical equation. For direct photoreduction to occur, the energy of thephoton causing the reaction must be higher than the free energy requiredto drive the reaction. To compare the energies, the photon energy can becalculated from Equation 2.

$\begin{matrix}{E_{photon} = \frac{hc}{\lambda}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

where h is Planck's constant, c is the speed of light, and λ is thephoton's wavelength. Photons available for solar collection aregenerally in the visible range, 380-750 nm.

Several potential redox materials have been examined for suitability inthe described systems using Equations 1 and 2. A sample of the resultsis presented in Table 1. All calculations presented assume a reactiontemperature of 500° C.

TABLE 1 Sample of calculations for redox material direct photoreductionΔG_(reaction) Required photon Balanced chemical equation[kJ/mol_(reactant)] wavelength Mn₂O₃ + hv → 2MnO + ½O₂ 30 <3940 nm 2Al₂O₃ + hv → 4Al + 3O₂ 717 <167 nm CoO + hv → Co + ½O₂ 176 <678 nmNiO + hv → Ni + ½O₂ 167 <718 nm

From this table, it can be seen that aluminum is not a good redoxmaterial option because its oxide cannot be reduced by visible light.Cobalt and nickel oxides cannot absorb from the entire red end of thespectrum but could capture most of the available energy.

Manganese oxide, alternatively, does not absorb the full energy ofvisible light for the reaction so some of the photon energy will beconverted to kinetic or thermal energy instead.

An ideal chemical energy storage (redox) material for the describedsystem will have four characteristics:

-   -   A high absorptivity in the UV and visible spectrum so the        maximum amount of light is absorbed.    -   An energy difference between the oxidized and reduced states        slightly smaller than the energy of visible spectrum photons so        the greatest amount of energy will be converted from        electromagnetic to chemical potential.    -   A low oxidation rate at lower temperatures and in the absence of        an ignition mechanism to minimize re-oxidation before the        desired time.    -   A high oxidation rate at the conditions of the power cycle to        maximize efficiency.

Several materials exhibiting these characteristics to varying degreeshave been previously identified. Generally, the best known materials aretransition metal oxides. For example, manganese oxides and cobalt oxideswith additions of iron oxide and aluminum oxide have previously beenidentified as prime candidates for direct photoreduction technologies.See, for example, General Atomics. “Thermochemical heat storage forconcentrated solar power based on multivalent metal oxides.” DOE ProgramReview, May 2011. http://www1.eere.energy.gov/solar/csp_pr2011.htmlaccessed on Dec. 19, 2011, which disclosure is incorporated herein inits entirety. However, heat loss management was identified as a problemin the above rotary kiln reactor study. Additionally, the fraction ofmaterial undergoing reduction was low, on the order of 3%, leading tohigh capital costs.

The systems and methods disclosed herein use solar photons to directlyphotoreduce the redox material. The energy does not go through a thermalstate in between the electromagnetic (solar photon) and the chemicalpotential state. In fact, the disclosed technology works best withmaterials that do not thermochemically dissociate below 1400° C., thedesired power cycle hot temperature, which is much hotter thanachievable with known CSP technologies.

The system and methods disclosed herein can be utilized to drive anytype of power generation cycles. The known power cycle most suited tooperation at efficiencies near or above 60% is however, an air-Braytoncycle or a variation thereof. A highly simplified diagram of onepossible representative and non-limiting power cycle layout 400 isillustrated in FIG. 4. The FIG. 4 example layout illustrates how thethermal and chemical heat sources described above could be integratedinto an air-Brayton power cycle 402 in conjunction with a steam Rankinebottoming cycle 404. As noted above, other power cycles could also beused. Certain advantages can be realized if the working fluid containsan oxidizing agent. For example, an open-loop supercritical carbondioxide or steam cycle could be used where the CO₂ or H₂O would bereduced to CO or Hz, respectively, which could then be used for liquidfuel generation or as fuel for fuel cells.

In the FIG. 4 embodiment, the upper open air Brayton cycle utilizes airas a working fluid and oxidizing agent. The air is initially compressedin compressor 406 which is driven by a mechanical connection to adownstream turbine 408. The compressed air from the compressor 406 isheated through heat exchange with the HTF in heat exchanger 106. Theheated and compressed air oxidizes the chemical energy storage materialin the oxidizer 112 and thus is further heated by direct contact orindirect heat exchange with the chemical energy storage material as itreleases heat during the exothermic oxidation reaction. The now hightemperature air drives one or more turbines 408 which in turn drive thecompressor 406 and one or more generators (not shown on FIG. 4) togenerate electrical energy.

The FIG. 4 embodiment also includes a lower steam Rankine bottomingcycle 404 receiving somewhat cooled air from the outlet of the turbine408. Heat is exchanged between the air and a secondary working fluid,for example steam, in a recuperator/heat exchanger 410. The heated steamthen drives a second turbine 412 or a second series of turbines which inturn drive one or more generators to generate electrical energy. Steamexiting the turbine 412 is condensed in a condenser 414 and pumped aswater back to the recuperator/heat exchanger 410 by pump 416.

One source of inefficiency for thermochemical and direct photoreductionchemical receivers is heat loss during the reduction stages. In acoupled thermal-chemical receiver as described herein, some or most ofthe heat losses from the redox material can be recaptured by the thermalreceiver and any residual heat stored in the redox material at theoutlet of the receiver can be transferred back to pre-heat the cool HTFentering the receiver.

A diagram illustrating a receiver design with improved heat lossmanagement is shown in FIG. 5. Incident solar radiation (shown as arrows502) is concentrated on the receiver 104 where some of the photons areabsorbed by the redox material (dots 504). A large part of the remainingincident photons are absorbed by the thermal receiver (illustrated aspanel 506). HTF is flowing within the panel 506 absorbing heat. Of thephotons absorbed by the redox material, some cause photoreduction whileothers directly heat the redox material. Some of this absorbed heat isradiated and lost to the environment but some is reabsorbed by thethermal receiver (illustrated by the dashed arrows 508). Similarly, someof the radiation losses from the thermal receiver will be reabsorbed bythe redox material. Because the redox material will be at a lowertemperature than the thermal receiver, the average surface temperatureseen by the environment will be lower than the thermal receiver surfacetemperature, thereby reducing total radiative thermal losses. Therefore,despite the relatively lower efficiency of the photoreduction step, thereceiver will still maintain high total efficiency.

The receiver embodiment of FIG. 5 couples a gravity fed curtain of redoxmaterial with a traditional cavity receiver tube sheet having HTFcooling. Alternatively, as shown in FIG. 6, the receiver element may beimplemented as a rotating cavity receiver 104 in which the walls arecooled by HTF and baffles are used to continuously drop the redoxmaterial 600 through the cavity space. In the FIG. 6 configuration, theredox material particles are contained in one or more rotating cavityreceivers 602. As the receiver 602 rotates, the particles 600 areagitated and fall through the space, absorbing solar radiation. Some ofthe photons will be absorbed by the reactor walls instead of theparticles and will be converted to heat. The reactor walls will becooled by the thermal pathway's heat transfer fluid 604. Thisconfiguration provides for minimized radiative and convective heatlosses and maximizes the conversion of solar energy to thermal energyand chemical potential.

A further advantage of the coupled thermal-chemical pathway system isthat the parallel thermal and chemical systems can be used to storeenergy over different time scales. Thermal CSP systems, such as moltensalt towers, provide for relatively low cost short term (day scale)thermal energy storage. For example, referring to FIG. 2, heated HTF maybe stored directly in a hot thermal storage system 116 operativelyassociated with the HTF conduit 108 receiving flow from the receiver 104before the heat exchanger 106. Alternatively, heated HTF could be usedto heat a separate thermal storage medium through heat exchange at thehot thermal storage system. Heat may then be provided to the HTF fromthe hot thermal storage system 116 during periods of low solar flux, inthe evening or during periods of cloud cover for example. Similarly,cooled HTF may be stored, or used to heat a separate heat storage mediumin a cold thermal energy storage system 118. The cold thermal energystorage system 118 could be operatively associated with the HTF conduit108 to receive flow from the heat exchanger 106 to the receiver 104 andused as above during periods of lower solar radiation.

The disclosed parallel chemical system enables longer term (seasonal)storage because the redox material is not stored at a high temperatureand therefore does not suffer from heat losses during storage. Forexample, as also shown in FIG. 2, oxidized or reduced redox material canbe stored for an extended period of time in an oxidized chemical storagematerial storage system 120 and reduced chemical storage materialstorage system 122 respectively. Both chemical storage systems 120 and122 could be operatively associated with the chemical energy storagematerial conduit with the oxidized material storage system beingdownstream from the oxidizer 112 and the reduced material storage systembeing downstream from the receiver 104.

One representative embodiment of the system 100 uses an aluminum silicon(AlSi) phase change material (PCM) as the HTF or in this example, theheat transfer material. AlSi PCM can reach higher temperatures thansteam or nitrate salts thereby providing improved receiver performanceHigher temperatures are advantageous because the system will performbest when the majority of the system heat requirement is supplied by ahigh efficiency thermal receiver.

As noted above, the disclosed system and methods may advantageously beimplemented in a power tower configuration consisting of a heliostatfield focused on a receiver on top of a tower structure. The AlSi PCM(or other suitable HTF) and the redox material will be transferred fromthe receiver to storage vessels or storage systems at the base of thetower. The PCM or other suitable HTF and redox materials may then betransferred to the power cycle as needed for electricity generation.

As noted above, one suitable but non-exclusive thermal-to-electricconversion system is an open air-Brayton power cycle with a steamRankine bottoming cycle. The inlet air will be compressed to highpressure, passed through a heat exchanger with the A1Si PCM or other HTFto heat it to medium temperatures, then passed through the oxidationchamber to oxidize the redox material and heat the air the very hightemperatures. The highly heated air will be used to power a turbine andelectric generator. The exhaust air will be used as the heat source fora typical steam bottoming Rankine cycle via a heat recovery steamgenerator.

The disclosed embodiments also include power generation methods, forexample the power generation method 700 illustrated in FIG. 7. The FIG.7 method includes several steps which may be performed in any suitableorder and which typically will be performed in a cyclical fashion. Themethod is initiated by providing a solar receiver configured to receiveconcentrated solar flux (step 702). Heat transfer fluid of any type isflowed, transported or otherwise brought into in thermal communicationwith the solar receiver where the HTF is heated with the concentratedsolar flux (step 704). The heated HTF is then flowed or transported fromthe solar receiver to a heat exchanger in a heat transfer fluid conduit(Step 706). In the heat exchanger heat is exchanged between the heatedheat transfer fluid and the working fluid of a power cycle (Step 708).

In a parallel cycle, a chemical energy storage (redox) material incommunication with the solar receiver is irradiated by the concentratedsolar flux thereby causing a quantity of the chemical energy storagematerial to be reduced (Step 710). The reduced chemical energy storagematerial is then flowed or transported between the solar receiver and anoxidizer in a chemical energy storage material conduit (Step 712). Inthe oxidizer, the chemical energy storage material is oxidized causingthe release of heat energy (Step 714). The released heat energy isexchanged with the working fluid of the power cycle (Step 716). Powermay then be generated with the heated working fluid of the power cycle(Step 718).

In summary, the systems and methods described herein featuring coupledthermal and chemical pathways will potentially achieve a higher overallsolar-to-electric conversion rate than any other known CSP technology.This will translate to direct capital cost and LCOE savings because eachcomponent can be proportionally smaller for a selected rate ofelectricity generation. A comparison of the disclosed technology to thestate of the art and developing CSP technologies can be found in Table2. All values in this table are estimates of target values and many havenot been proven commercially to date.

TABLE 2 Comparison of proposed technology to state of the art anddeveloping technologies (estimated target values) Power Solar fieldReceiver Storage Cycle Overall efficiency efficiency efficiencyefficiency Efficiency Molten salt 50-60% 80-90% 97% 40-45% 16-24% Air50-60% 70-80% n/a 50-55% 18-26% Saturated 50-60% 85-90% n/a 30-35%13-19% Steam Superheated 50-60% 80-85% n/a 40-45% 16-23% Steam Solidparticle 50-60% 60-70% 97% 45-50% 13-20% Coupled 50-60% 70-85% 97%55-60% 19-30% thermal and chemical

In addition to high system efficiency at low cost, the coupledchemical-thermal pathway systems and methods offer two other significantbenefits. First, as described in detail above, the two energy pathwaysoffer two means of energy storage. The thermal pathway can utilize anyexisting thermal storage system for short term storage. This is animportant advantage that CSP holds over wind and photovoltaictechnologies because it allows CSP plants to match demand while reducingthe LCOE. The described system can also couple inexpensive short termstorage with long term chemical storage to match seasonal demand Oncereduced, the redox material can be stored in an inert environment forvery long periods of time and used for power production as needed. Thiswill further allow CSP to meet grid demands during times when verylittle renewable generation is available.

The second additional benefit is the ability to produce syngas. Insteadof using air as the oxidant in power cycle, the redox material could becombusted with carbon dioxide or steam to produce carbon monoxide orhydrogen. Together, these two gases constitute syngas which can be usedto create liquid fuels. This process would possibly decrease theelectric generation capacity of the system but may be a relativelyefficient way to produce renewable carbon-neutral fuels.

Various embodiments of the disclosure could also include permutations ofthe various elements recited in the claims as if each dependent claimwas a multiple dependent claim incorporating the limitations of each ofthe preceding dependent claims as well as the independent claims. Suchpermutations are expressly within the scope of this disclosure.

While the invention has been particularly shown and described withreference to a number of embodiments, it would be understood by thoseskilled in the art that changes in the form and details may be made tothe various embodiments disclosed herein without departing from thespirit and scope of the invention and that the various embodimentsdisclosed herein are not intended to act as limitations on the scope ofthe claims. All references cited herein are incorporated in theirentirety by reference.

What is claimed is:
 1. A concentrating solar power system comprising: asolar receiver configured to receive concentrated solar flux; heattransfer fluid in thermal communication with the solar receiver suchthat concentrated solar flux heats a quantity of the heat transferfluid; a heat exchanger in thermal communication with the heat transferfluid, the heat exchanger providing for heat exchange between the heattransfer fluid and a working fluid of a power cycle; a heat transferfluid conduit providing for the flow or transport of heat transfer fluidbetween the solar receiver and the heat exchanger; chemical energystorage material in communication with the solar receiver such thatconcentrated solar flux reduces a quantity of the chemical energystorage material; an oxidizer in communication with the chemical energystorage material, the oxidizer providing for the oxidation of thechemical energy storage material and further providing for heat exchangebetween the chemical energy storage material and the working fluid ofthe power cycle; and a chemical energy storage material conduitproviding for the flow or transport of chemical energy storage materialbetween the solar receiver and the oxidizer.
 2. The concentrating solarpower system of claim 1 further comprising a thermal energy storagesystem operatively associated with the heat transfer fluid conduit. 3.The concentrating solar power system of claim 2 further comprising: ahot thermal energy storage system receiving heated heat transfer fluidfrom the solar receiver; and a cold thermal energy storage systemreceiving cooled heat transfer fluid from the heat exchanger.
 4. Theconcentrating solar power system of claim 1 further comprising: areduced chemical energy storage material storage system operativelyassociated with the chemical energy storage material conduit andreceiving reduced chemical energy storage material from the solarreceiver; and an oxidized chemical energy storage material storagesystem operatively associated with the chemical energy storage materialconduit and receiving oxidized chemical energy storage material from theoxidizer.
 5. The concentrating solar power system of claim 1 wherein theheat transfer fluid comprises one or more of water, a solid salt; amolten salt, a solid metal; a molten metal and an oil.
 6. Theconcentrating solar power system of claim 1 wherein the heat transferfluid comprises an aluminum silicon phase change material.
 7. Theconcentrating solar power system of claim 1 further comprising: a towersupporting the solar receiver; and a heliostat field having heliostatspositioned to focus sunlight on the receiver.
 8. The concentrating solarpower system of claim 1 wherein the power cycle comprises: an open airBrayton upper power cycle; and steam Rankine bottoming cycle.
 9. Theconcentrating solar power system of claim 1 wherein the working fluid ofthe power cycle contains an oxidizing agent.
 10. A power generationmethod comprising: providing a solar receiver configured to receiveconcentrated solar flux; heating a heat transfer fluid in thermalcommunication with the solar receiver with the concentrated solar flux;flowing or transporting the heat transfer fluid between the solarreceiver and a heat exchanger in a heat transfer fluid conduit;exchanging heat between the heated heat transfer fluid and a workingfluid of a power cycle within the heat exchanger; reducing a chemicalenergy storage material in communication with the solar receiver byirradiating the chemical energy storage material with concentrated solarflux; flowing or transporting the reduced chemical energy storagematerial between the solar receiver and an oxidizer in a chemical energystorage material conduit; oxidizing the reduced chemical energy storagematerial in an oxidizer, the oxidizer further providing for heatexchange between the chemical energy storage material and the workingfluid of the power cycle; and generating power with the working fluid ofthe power cycle.
 11. The method of claim 11 further comprising storingheat transfer fluid in a thermal energy storage system operativelyassociated with the heat transfer fluid conduit.
 12. The method of claim11 further comprising: storing heated heat transfer fluid received fromthe solar receiver in a hot thermal energy storage system; and storingcooled heat transfer fluid received from the heat exchanger in a coldthermal energy storage system.
 13. The method of claim 11 furthercomprising: storing reduced chemical energy storage material receivedfrom the receiver in a reduced chemical energy storage material storagesystem; and storing oxidized chemical energy storage material receivedfrom the oxidizer in an oxidized chemical energy storage materialstorage system.
 14. The method of claim 11 wherein the heat transferfluid comprises one or more of water, a solid salt; a molten salt, asolid metal; a molten metal and an oil.
 15. The method of claim 11wherein the heat transfer fluid comprises an aluminum silicon phasechange material.
 16. The method of claim 11 further comprising:providing a tower to support the solar receiver; and providing aheliostat field having heliostats positioned to focus sunlight on thereceiver.
 17. The method of claim 11 further comprising generating powerwith a power cycle comprising: an open air Brayton upper power cycle;and steam Rankine bottoming cycle.
 18. The method of claim 11 furthercomprising oxidizing the reduced chemical energy storage material in theoxidizer with a oxidizing agent in the working fluid of the power cycle.